Many enclosures that contain sensitive instrumentation must maintain very clean environments in order for the equipment to operate properly. Examples include enclosures with sensitive optical surfaces or electronic connections that are sensitive to particles and gaseous contaminants which can interfere with mechanical, optical, or electrical operation. Other examples include data recording devices such as computer hard disk drives that are sensitive to particles, organic vapors, and corrosive vapors. Still others include enclosures for processing, transporting or storing thin films and semiconductor wafers. Also included are electronic control boxes such as those used in automobiles and industrial applications that can be sensitive to particles, moisture buildup, and corrosion as well as contamination from fluids and vapors. Contamination in such enclosures originates from both inside and outside the enclosures. For example, in computer hard drives, damage may result from external contaminates as well as from particles and outgassing generated from internal sources. The terms “hard drives” or “hard disk drives” or “disk drives” or “drives” will be used herein for convenience and are understood to include any of the enclosures mentioned above.
One serious contamination-related failure mechanism in computer disk drives is static friction or “stiction”. Stiction results from the increased adhesion of a drive head to a disk while the disk is stationary plus increased viscous drag parallel to the head-disk interface. Contaminants on the disk change the surface energy and the adhesive forces between the head and disk; this also causes stiction. Vapors that condense in the gap between the head and disk can cause stiction. High-density disks are more sensitive to contamination-caused stiction because they are smoother and only thin layers of lubricants are present. Further exacerbating these effects are the newer lower energy, lower torque motors being used in smaller disk drives for portable computers and consumer applications.
Another serious contamination-related failure mechanism in computer disk drives is head crashes. Head crashes can occur when particles get into the head disk interface. High density drives may have less than 30 nanometer flying heights or spacing between the head and disk during operation and typically have disks rotating 7200 revolutions per minute or greater. Even submicron-sized particles can be a problem, causing the head to crash into the particle or the disk after flying over a particle, bringing the drive to an abrupt failure mode. Particles can also adversely affect data integrity and mechanical reliability of a drive, sometimes referred to as thermal asperity.
Internal particulate filters, or recirculation filters, are well known. These filters are typically pieces of filter media, such as expanded PTFE membrane laminated to backing material such as a polyester nonwoven, or “pillow-shaped” filters containing electret (i.e., electrostatic) filter media. They may be pressure fit into slots or “C” channels and placed in the active air stream such as near the rotating disks in a computer hard disk drive or in front of a fan in electronic control cabinets, etc. Alternatively, the recirculation filter media can be framed in a plastic frame. In very small drives, these small standup recirculation filters are so very small and the air being circulated by the very small disks is so low, that the filter effectiveness of these filters is minimal at best.
Internal adsorbent filters are also well known. A sorbent filter may be constructed of powdered, granular or beaded sorbent or sorbent mixture encapsulated in an outer expanded PTFE tube. Such a filter is manufactured by W. L. Gore & Associates, Inc., Elkton, Md., and is commercially available under the trademark GORE-SORBER® module. A second well known internal adsorbent assembly incorporates a layer of adsorbent, such as activated carbon/PTFE composite, between an encapsulating filter layer and layer of pressure sensitive adhesive that helps encapsulate the adsorbent as well as provides a means of mounting the adsorbent assembly on an interior wall in the enclosure.
A third internal adsorbent assembly incorporates a layer of adsorbent such as activated carbon/PTFE composite between two layers of filter media or is alternately wrapped in a layer of filter media and can be installed between slots or “C” channels much the way a recirculation filter is installed. These filters have minimal airflow through the filter.
All of these internal adsorbent filters adsorb vapor phase contaminants well, but they do not filter particulates very well. They can collect particles by some impaction of particles onto the filter (i.e., by having the larger particles impacting or colliding with the adsorbent filter as particle-laden air speeds around the filters) or by diffusion of particles onto the filter. However, they do not perform nearly as well as standard recirculation filters that work by a combination of sieving (mechanically capturing particles too large to pass through the pore structure of the filter), impaction (capturing particle too large to follow the bending air streams around filters or the fibers of the filter), interception (capturing particles that tend to follow the air streams, but are large enough to still intercept a filter fiber or in other words those particles with a diameter equal to or less than the distance between the fiber and the air stream line), and diffusion (capturing smaller particles buffeted about by air molecules in a random pattern and coming into contact with a filter fiber to become collected).
A multifunction filter providing a breather filter and a recirculation filter with optional; adsorbents can solve many of the problems associated with the previous filters. A multifunction filter is described in U.S. Pat. No 6,395,073 to Dauber. This is an adequate solution when the space can be found for placing such a combination filter.
Disk or shroud Filters are also known. Such filters are placed under the disk, or in close proximity to its perimeter. Because typically carpet and shroud filters use fibrous media. It is difficult to position this fibrous media very near the computer disk, because the fibers can extend from the filter and contact the computer disk. This may cause more particles to be generated and deposited onto the hard disk, which can lead to a catastrophic failure of the hard drive.
However, the limited space in smaller drives often necessitates that these filters be placed either directly over or under the disks. Moreover, particularly in smaller drives where the disks are very close to the top cover and base plate, these filters can perform better in these locations than in standard upright locations traditionally used in larger drives. In multifunction filters installed within small drives, or within any drive where the clearance between the filter and the drive components is small, fibers protruding from the filter present problems. The clearance between a hard disk and the filter may be less than 0.5 mm and a filter must fit within this thickness as well as leave clearance for the disks to spin without possible contact with the filter.
What is needed is a recirculation filter material with low fiber height to permit the material to be used as a carpet or shroud filter.
Accordingly, the present invention provides a reduced fiber carpet/shroud filter material that can filter the air of particles to prevent fibers from contacting and interfering with any moving parts within the enclosure. The invention also provides a carpet or shroud filter with reduced fiber height.
The invention may optionally include adsorbents to filter gaseous contaminants from the enclosure.